It is understood the party has informed the commissioner about the alleged breach and that it intends to submit a full report. Suspicions about the breach prompted party officials to shut down its key canvassing software.

The two tools shut down on Wednesday were Contact Creator, which is a party canvassing software, and Organise. Content Creator identifies specific people in a constituency that candidates can target with messages.

The party’s general secretary, Jennie Formby, told staff in a message that “data held by the party, including data within Contact Creator and other systems used for election or other campaigning work, may only be accessed by individuals who are authorised to access it, and may be used only for purposes authorised by the party as data controller.”

She added: “Much of the data held on our systems tends to reveal individuals’ political opinions and is therefore ‘special category’ data, benefiting from enhanced protection under the legislation.”

Ryan told the Guardian: “I have no idea what Jennie Formby is talking about. Neither I nor my office have accessed or used any Labour party data since I resigned the Labour whip and my membership of the Labour party.”

A Labour party spokesperson said: “The Labour party became aware of attempts to access personal data held on the party’s systems for unauthorised use. Personal data the party holds about individuals is protected by law, under the GDPR and Data Protection Act 2018.

“We are aware that the information commissioner is taking an increasingly serious view of misuse of personal data and requires a data controller to take reasonable and proportionate steps to ensure the security of data held on its systems. The Labour party takes our data protection obligations extremely seriously.”

Chris Leslie, also of the Independent Group, insisted after the alleged breach was first reported that he and his colleagues would not seek to use party resources and earlier accused Labour of “throwing mud” to discredit their departed MPs.

Leslie said they had tried to comply with all the rules. “We did think through in our checklist of things that had to be done, obviously, if you’d resigned your Labour membership, you don’t have access to Labour facilities and data and all those other things,” he said.

“Obviously while you’re still a Labour MP and you’re still a member of the Labour party you have access and use of all of that data as it was. This is a process of separation and if the focus of the party is going to be to pick holes in the process of that separation, so be it. But we’re trying our best to do this in as decent a way as possible.”

Other members of the Independent Group said they were not aware Ryan was planning to join their number until shortly before she resigned.

The newly formed group, whose members also include Chuka Umunna, Luciana Berger, Anna Soubry and Sarah Wollaston, has already built up a significant social media presence, gaining more Twitter followers since its formation on Monday than the leftwing grassroots group Momentum.